1. Field of the Invention
The present invention relates to dielectric filters primarily for use in the microwave band and to communication apparatuses using the same.
2. Description of the Related Art
A conventional dielectric filter having a conductor film on a dielectric block is constructed by forming a substantially rectangular dielectric block, which has a plurality of holes therein, and by forming an outer conductor on outer surfaces of the dielectric block. Inner conductors are formed in the holes while certain portions of the holes near apertures of the holes are devoid of inner conductor, forming respective gaps in the inner conductors.
With this arrangement, the dielectric block, the inner conductors, and the outer conductor operate as resonators in a transverse electromagnetic (TEM) mode. The resonators are combline-coupled by stray capacitance formed in the sections devoid of inner conductor. In the dielectric filter having the above arrangement, an attenuation pole (which will be referred to as a coupling pole) is generated by the coupling between the resonators. The attenuation pole is employed to provide a steep attenuation curve between a pass band and a lower stop band, or between the pass band and a higher stop band.
Since the resonators are combline-coupled by the stray capacitance generated in the sections devoid of inner conductor, the attenuation pole is generated. When the attenuation pole is generated in the lower stop band near the pass band, the attenuation characteristics become steeper in the band below the pass band. In contrast, the attenuation characteristics between the pass band and the higher stop band are not improved or controlled. If steep attenuation characteristics are required between the pass band and both the lower and higher stop bands, it is necessary to increase the number of poles generated by the resonators, or to employ additional structures to create another attenuation pole. As a result, the overall structure of the dielectric filter becomes complex and is difficult to miniaturize.
In the above dielectric filter having the outer conductor on the outer surfaces of the substantially rectangular dielectric block, resonant modes, such as a TE101 mode, in addition to the TEM mode, which is the fundamental mode, are generated by the dielectric block and the outer conductor.
The resonant modes other than the fundamental TEM mode which is intended for actual use are regarded as spurious modes, and efforts have been made to suppress these spurious modes. For example, Japanese Unexamined Patent Application Publication No. 8-51301 proposes to partially trim the outer conductor on the end surface of the dielectric block near the sections devoid of inner conductor, thus adjusting the TE-mode resonant frequency. Specifically, the TE-mode resonant frequency is separated from the TEM-mode resonant frequency so as to suppress the influence of the TE mode.
Accordingly, it is an object of the present invention to provide a dielectric filter, having a simplified overall structure, for eliminating or minimizing the above problems by generating an attenuation pole other than the above-described coupling pole, and a communication apparatus using the same.
According to one aspect of the present invention, there is provided a dielectric filter including a substantially rectangular dielectric block which has a plurality of holes therein; an outer conductor on outer surfaces of the dielectric block; and inner conductors in the holes, the inner conductors having gaps which are devoid of inner conductor near apertures of the holes. The dielectric block, the inner conductors, and the outer conductor form a plurality of TEM-mode resonators. The TEM-mode resonators are coupled to each other so as to generate an attenuation pole in a band below a pass band. A section devoid of outer conductor is provided on part of an edge portion around an opening surface of the dielectric block in which the holes open.
Since the outer conductor is formed on the outer surfaces of the dielectric block, the, entire dielectric block and the outer conductor operate as a TE-mode resonator. The section devoid of outer conductor is provided to generate the TE mode having a low resonant frequency. The TE-mode resonance and the TEM-mode resonance are combined to generate an attenuation pole in a band higher than and close to the pass band.
The opening surface may be a surface nearer to the gaps. Thus, even when the section devoid of outer conductor is formed on the edge portion around the opening surface, the TEM mode maintains the greatest electrical field strength at the gaps, so that the characteristics of the TEM-mode resonators are not significantly influenced. As a result, the TE-mode resonant frequency can be brought closer to the TEM-mode resonant frequency in a more efficient manner. Since the section devoid of outer conductor is not formed on a short-circuit surface having a high current density, the quality factor (Q) of the resonators will not be reduced.
The section devoid of outer conductor may be provided on the edge portion along at least a long side of the opening surface of the dielectric block. Thus, the TE mode having a low resonant frequency can be easily generated even when the section devoid of outer conductor is relatively small.
The section devoid of outer conductor may be formed by first forming a projection at an edge portion of the dielectric block, then forming the outer conductor on the dielectric block, and then removing the outer conductor from the projection. With this method, the section devoid of outer conductor is easily obtained by simply removing the projection, subsequent to forming the outer conductor on the outer surfaces of the dielectric block including the edge portion having the projection.
According to another aspect of the present invention, there is provided a communication apparatus including the above dielectric filter for use in a high frequency circuit for microwave band signals.